Gnashing: Grinding the teeth. More formally called "bruxism."
Gnashing, or bruxism, is due to clenching of the teeth other than in chewing and is associated with forceful lateral or protrusive jaw movements. This results in grinding or rubbing the teeth together.
Gnashing usually occurs during sleep. It is sometimes so excessive that it damages the occlusal surfaces of the teeth, particularly the molar teeth, and may contribute to the temporomandibular joint (TMJ) syndrome.
The word "gnashing" comes from the Old Norse and likely is an example of onomatopoeia, an action named for the sound associated with it.